Lapping: Appearance of a denser color or higher gloss where wet
and dry layers overlap during paint application.

Possible Cause:

Failure to maintain a "wet edge" when applying paint.

Solution:

Maintain a wet edge when painting by applying paint toward
the unpainted area and then back into the just-painted
surface. This technique (brushing from "wet to dry",
rather than vice versa) will help produce a smooth,
uniform appearance. It is also wise to minimize the area
being painted and plan for interruptions at a natural
break, such as a window, door or corner (especially
important when applying stain to bare wood). Alkyd paints
generally have superior wet edge properties.